The present invention relates to a magnetic recording medium and a magnetic storage apparatus, specifically to a magnetic recording medium having a recording density of 50 Gbits or more per square inch and to a magnetic storage apparatus incorporating the same.
In recent years, an areal recording density of a magnetic disk device as an external storage apparatus of a computer is increased by 100% per year. However, as the areal recording density is increased, a problem that data magnetically recorded is erased by circumferential heat, that is, a so-called thermal fluctuation, has become obvious. Accordingly, the conventional longitudinal recording method has been considered to be difficult to achieve the areal recording density exceeding 50 Gbits per square inch.
On the other hand, unlike the longitudinal recording method, a perpendicular recording method has a feature that, as a linear recording density is increased, a demagnetizing field acting between adjacent bits is decreased to stabilize recorded magnetization. Accordingly, the perpendicular recording method is considered to be one of the effective means for exceeding the thermal fluctuation limit of the longitudinal recording method.
In the perpendicular recording method, a combination of a single pole type head and a double-layer perpendicular medium composed of a soft magnetic underlayer and a perpendicular recording layer is effective in realizing high density recording. However, since the double-layer perpendicular medium includes the soft magnetic underlayer of a high saturation magnetic flux density (Bs), following problems have been pointed out: a stray field generated from domain walls of the soft magnetic underlayer is observed as spike noises, or recorded magnetization disappears by displacement of domain walls of the soft magnetic underlayer. As a method for solving these problems, for example as disclosed in Japanese Patent Laid-Open Nos. 07(1995)-129946 and 11(1999)-191217, it has been proposed that a hard magnetic pinning layer is provided between the soft magnetic layer and a substrate so that magnetization directions of the soft magnetic layer are aligned with one direction. As disclosed in Japanese Patent Laid-Open No. 06(1994)-103553, a method has been proposed, in which the displacement of domain walls of the soft magnetic layer is suppressed by an exchange coupling with an antiferromagnetic layer having magnetic spins aligned with each other.
However, in the method of aligning magnetization directions of the soft magnetic underlayer by use of the hard magnetic pinning layer, magnetic domains having an opposite magnetization direction are likely to be formed around inner and outer edges a disk substrate, and spike noises therefrom are observed. On the other hand, the method of suppressing the displacement of domain walls of the soft magnetic underlayer by use of the antiferromagnetic layer has an effect for preventing the disappearance of the recorded magnetization, which is caused by the displacement of domain walls, but cannot prevent the spike noises attributable to the domain walls.